honestly dude, i just got a new PSU. I didnt need one for SLI etc. so I settled on a 650W unit. I first bought an Ultra LSP650 but it was noisy and i read horrible reviews so I took it back and got the Corsair TX650 - I couldn't be happier. The difference between the corsair and the ultra is night and day. Make sure you get a quality unit - at least tier 1 or 2 from the following page: http://www.eggxpert.com/forums/thread/323050.aspx
The Antec will only work if you have an Antec case. It will not fit in any other cases.
The PC Power and Cooling is a great power supply and at $129 after rebate is a good deal. You will not find a better power supply of that size for the same price unless you catch a crazy good sale somewhere.
No actually you do not need a big power supply for 2 x HD 7850s.
From AMD's website:
"■PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard
■500W (or greater) power supply with one 75W 6-pin PCI Express power connector recommended
■600W power supply (or greater) with two 75W 6-pin PCI Express power connectors recommended for AMD CrossFire™ technology."
That is assuming you are buying a no name power supply as well. A high quality power supply puts out more than it's rated power. Now there is nothing harmful about overpowering your computer, except to your wallet of course.
So we can save you some money. Any of these 4 are very good and provide plenty of power for your computer even when you add a second card. Even if you overclock everything. The OCZ, Antec and Seasonic are modular.